The title asks a question, and the poem never answers it. "What Changed" gives us four stanzas of before and after, but no event, no culprit, no moment when the light went out. The speaker simply reports the result. The central idea is that happiness belonged to a state of not-knowing, and that losing it has made even ordinary joys, like looking at the stars, hard to bear.
The first stanza is the oddest, and the most revealing. "I used to be happy / But then I was naïve." We expect "but then" to introduce something that happened to the speaker. Instead it introduces a verdict on the speaker's past self. The old self "would act stupid / And think myself brave," and the harshness is striking. Bravery is recast as a belief ("think myself brave") rather than a fact, and happiness is bundled together with foolishness, as if the two couldn't be separated. This is also the one place where the rhyme slips. "Naïve" and "brave" share only a final consonant sound, so the pairing is a near-miss. That fits a stanza about misjudging oneself, though it may be an accident.
After that, the poem settles into a steady pattern. Each stanza sets a "used to" against a "now" (the final one swaps in "But that was before"), and the rhyme scheme is plain, with the second and fourth lines chiming: bright/light, gleam/scream, own/groan. The short lines bounce along with a light, singsong beat, almost like a children's verse. That cheerfulness of sound sits uneasily against what is being said, as though the poem still carries the rhythm of the happier self while describing its loss.
Light is the poem's main image. The stars "glimmer ever so bright," and "ever so" has a childlike ring, the voice of someone who hasn't yet learned to be guarded. The eyes then borrow that brightness: they "sparkle," "dance," "gleam." When the speaker "draws the curtains / To shut out all light," the sky is replaced by a piece of household fabric, which makes the retreat feel small, private and domestic. The word "all" matters too. The speaker isn't avoiding one painful sight but light in general. Notice that the eyes never appear in the present tense. The poem lists their old liveliness and then simply moves on, as though there is nothing left to describe.
From there the poem works downward through the face. Eyes give way to a bitten lip, and the lip gives way to lips that "part to groan." The last two stanzas are the most carefully balanced. The smile once came "on its own," with no effort, whereas now the speaker must "try not to scream." Joy used to be automatic, and pain has to be managed. The final rhyme underlines this, because "own" and "groan" share a sound but mean opposite things. The mouth is still acting involuntarily; only the result has changed. There is also a small, telling difference between the two sounds. The scream is held back, but the groan gets out. Judging by the phrasing, the speaker's self-control is not quite winning.
What makes the poem significant is its restraint. It uses simple words and almost no figurative language beyond light and dark, and it never explains or asks for sympathy. Because it withholds the cause of the change, readers are free to fill that gap with their own experience. Many people recognise the feeling of having lost something without being able to point to the day it happened. The poem also leaves an uncomfortable idea hanging: that knowing more, or being hurt, may cost a person the ability to feel carefree. It does not offer comfort or a turn toward hope. It ends on a groan, and that honesty is exactly what gives it force.